1. Field of the Invention
The invention relates to a method for determining the cylinder interior pressure of an internal combustion engine.
2. The Prior Art
Interior pressure of the cylinder is a decisive variable for assessing a combustion process. This also holds true for combustion processes in which a homogeneously pre-mixed fuel/gas mixture is ignited by auto ignition. This combustion process is frequently referred to as HCCI (homogeneous charge compression ignition), homogeneous compression ignition, homogeneous auto-ignition, or, for example for gasoline as the fuel of homogeneous compression ignition, as CAI, controlled auto ignition. In order to achieve homogeneous compression ignition, a sufficiently high energy level must be present in the cylinder. This can be brought about, for example, by a high compression pressure and/or by means of a sufficient amount of hot residual gas.
Control of the parameters that influence auto ignition, such as compression, exhaust gas reflux rate, injection time, fuel distribution, air filling, etc., is very complex. In this connection, setting variables can be the control of inlet and outlet valves of the internal combustion engine, for example, which can be variably controlled with regard to their stroke and their on time. Both the compression and the exhaust gas reflux rate, in the case of internal exhaust gas reflux, can be influenced by variable on times. Additional influence parameters can be the setting of an exhaust gas reflux valve, an adjustable compression ratio, or the injection parameters. In this connection, the operating mode of homogeneous auto ignition cannot be represented in all operating points, but rather these are mostly limited to the range of partial load. It is difficult to manage the process, particularly in the marginal ranges and at dynamic operating transitions. Because of the leanness in the case of homogeneous auto ignition, the torque that can be given off is limited, and there is the risk of combustion failures. Enrichment of the mixture to increase the torque that can be given off leads out of the range of homogeneous auto ignition. Furthermore, it is problematic that a drastic increase in the cylinder pressure gradient can be seen at the rich limit of the process, which increase can bring about damage to the internal combustion engine. Furthermore, a severe cylinder pressure gradient has effects on the acoustics of the internal combustion engine. It is therefore necessary to observe the cylinder interior pressure, particularly for the control of an internal combustion engine operated with homogeneous auto ignition, for safety aspects, and this is furthermore suggested for monitoring and control of the combustion process.
Direct measurement of the cylinder interior pressure can take place with additional sensor systems, whereby cylinder pressure sensors are complicated and expensive for standard applications. Measurement devices for direct measurement of the cylinder pressure are therefore not installed in standard vehicles. This results in the need to calculate the cylinder interior pressure from other variables that can easily be measured or are already available in the control device. The non-uniformity of rotation of the crankshaft offers an approach for calculating the cylinder interior pressure; it can be measured by measuring the differences in speed of rotation and the derived angular velocity and angular acceleration at the crankshaft, by the angle mark transducer installed in standard vehicles.
A method for determining the torque of internal combustion engines is previously known from DE 44 45 684 A1. Proceeding from a precise measurement of the angular velocity or angular acceleration, the torque that is given off is determined by the torque balance at the crankshaft. In this connection, determination of the torque takes place under simplified conditions.
Furthermore, a method for determining the indicated torque from the angular velocity or acceleration of the crankshaft and determining the torque sum from the free torque and the mass torque of the oscillating masses is described in German Patent No. DE 199 31 985 A1.
German Patent No. DE 102 56 106 A1 describes carrying out compensation of the rotary sensor error or of the influence of the charging pressure, in order to improve the accuracy of the calculation of the torque.
Determining the torque from the angular velocity measurement is described in International Application No. WO 93/22648 A1, which teaches that internal rotational vibrations of the drive train can be modeled as elastically connected individual masses, which makes measurement of each of the angular velocities of the individual masses necessary. However, this is possible, in the drive train, only with additional sensor systems.